feat(pmic): Add config option to disable low battery check

We have encountered a device where the connection from the PMIC's ADMUX
pin to the MCUs ADC is severed and thus the low battery check would
trigger immediately.

For making such devices still usable, add a config option to disable the
low battery check entirely.

Fixes: #227
2 jobs for !464 with rahix/low-bat-disable in 1 minute and 29 seconds (queued for 2 seconds)
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